Process of producing chlorine dioxide

ABSTRACT

The invention relates to a process of producing chlorine dioxide using, in a first alternative atmospheric pressure and hydrogen peroxide in the reaction medium optionally with an electrolytic cell. In a second alternative, either atmospheric or subatmospheric pressure is used in the reaction medium together with an organic or inorganic reducing agent and an electrolytic cell.

CROSS REFERENCE TO RELATED APPLICATION

This disclosure is a continuation of patent application Ser. No.08/120,814, filed Sep. 15, 1993, now U.S. Pat. No. 5,487,881, which is acontinuation-in-part patent application of prior patent application08/023,561, filed Feb. 26, 1993, now U.S. Pat. No. 5,380,517.

BACKGROUND OF THE INVENTION

The present invention relates to a process of continuously producingchlorine dioxide. The process comprises reducing chlorate in an acidicreaction medium and, optionally, circulating generator solution betweena chlorine dioxide generator and an electrochemical cell. When using anelectrochemical cell, the process is performed without crystallizationof sulfate and without formation of any solid by-products. The processcan also be performed under a wide range of pressures, from vacuum toabove atmospheric pressure. In one process alternative, a wide range ofreducing agents can be used, including methanol and hydrogen peroxide.

Chlorine dioxide used in aqueous solution is of considerable commercialinterest, mainly in pulp bleaching, but also in water purification, fatbleaching, removal of phenols from industrial wastes, etc. It istherefore desirable to provide processes in which chlorine dioxide canbe efficiently produced. Considerable research is also directed to thehandling of by-products such as chlorine and mineral acid salts.

There are numerous different processes for chlorine dioxide production.Most processes in commercial use involve reaction of alkali metal (e.g.sodium) chlorate in an acidic medium with a reducing agent. The acidityis generally provided by sulfuric acid. Alkali metal chlorate andsulfuric acid are brought continuously to a reaction vessel to which airand the reducing agent are introduced, usually into the bottom of thevessel. Then chlorine dioxide and air leave from the top of the reactionvessel and a depleted reaction solution is withdrawn for furthertreatment. It is common to use more than one vessel whereby the depletedreaction solution from the first vessel is brought to a second (andsubsequent) vessel together with air and reducing agent for furtherconversion of the remaining chlorate. The reaction in the reactionvessel(s) is carried out at about atmospheric pressure. Reducing agentsused in this type of reaction are sulfur dioxide (the Mathiesonprocess), methanol (the Solvay process) and chloride ions (the R-2process). The basic chemical reaction involved in the process withchloride ions can be summarized with the formula:

    ClO.sub.3.sup.- +Cl.sup.- +2 H.sup.+ →ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O                                                [1]

The other reducing agents are indirect reducing agents, the directreaction between chlorate ions and methanol or sulfur dioxide being veryslow. The direct reducing agent in these cases are chloride ionsreacting according to [1]. The chlorine produced is then reacting withmethanol to regenerate chloride ions according to the formula:

    CH.sub.3 OH+3 Cl.sub.2 +H.sub.2 O→6 Cl.sup.- +CO.sub.2 +6 H.sup.+[ 2]

or with sulfur dioxide according to the formula:

    SO.sub.2 +Cl.sub.2 +2 H.sub.2 O→2 HCl+H.sub.2 SO.sub.4[ 3]

As is evident from reaction [1] a large amount of chlorine is producedas a by-product when chloride ions are used as reducing agent. To reducethe amount of chlorine by-product formed in the process, methanol hasbeen used instead of chloride ions as the reducing agent. However, withmethanol and also with sulfur dioxide, a certain amount of chlorine isproduced since chloride ions are involved in the reduction process. Itis also common in these processes to add a small amount of chlorideions, in the form of sodium chloride or hydrochloric acid, to increasethe efficiency. Formerly, the chlorine by-product has been utilized inpaper mills, but due to increased environmental demands there is adecreasing need for chlorine.

The change over from chloride ions to methanol as the reducing agentalso resulted in the disadvantage of formation of by-products other thanchlorine in the reaction system. The reaction according to reaction [2]above represents only the theoretical methanol oxidation. However, inpractical production, inefficiencies in the methanol oxidation bringabout the formation of formaldehyde and formic acid and probably alsoethers and esters along with carbon dioxide. It could be expected thatreactions can occur in the bleaching train with these by-productsthereby resulting in chlorinated organic compounds.

Besides the drawback of formation of chlorine and other by-products, theold R-2, Solvay and Mathieson processes also have the disadvantage oflow efficiency and low production rates. The efficiency for a normalMathieson process calculated as chlorate transformed into chlorinedioxide is typically not more than about 88%.

To increase the efficiency of these processes it has been suggested torun the processes in a single vessel under subatmospheric pressure.Chlorine dioxide is then generated continuously together with theevaporated aqueous reaction medium. The alkali metal sulfate by-productis crystallized. This process is disclosed e.g. in U.S. Pat. No.4,081,520. This process and similar "single vessel process" ("SVP"process) technologies generally increase the efficiency to acceptablelevels while maintaining low levels of chlorine effluent. Patents issuedafter the above mentioned patent describe different embodimentsattempting to optimize the process with as low chlorine production aspossible.

Another reducing agent suggested in the prior art for chlorine dioxideproduction is hydrogen peroxide. U.S. Pat. No. 2,332,181 discloses abatch process for chlorine dioxide production of substantially purechlorine dioxide with respect to chlorine with hydrogen peroxide as thereducing agent. The process must be run at a low temperature and withlow concentrations in the reactor to avoid explosive decomposition.Other patents suggest a combination of hydrogen peroxide and chlorideions as the reducing agent. This combination has the disadvantage ofchlorine formation. U.S. Pat. No. 5,091,167 teaches that it is possibleto produce chlorine dioxide continuously with high efficiency withhydrogen peroxide as the reducing agent in a chlorine free process withthe SVP technology.

However, there is still a need for developing chlorine dioxide processesat atmospheric pressure with good efficiency and production rate butwith reduced production of chlorine by-product as well as otherby-products. For example, there are a large number of existing plantswith atmospheric pressure generators having poor efficiency and capacitylimitations. With increasing demand for chlorine dioxide bleaching,improvements of these plants would be of considerable interest. Also,for the installation of new plants, the atmospheric pressure processoffers a low investment cost for the chlorine dioxide generator.

Another drawback of known chlorine dioxide processes is the formation ofsome form of sodium sulfate as a by-product which has to be removed fromthe reactor, either in the form of a solid saltcake or as waste acid. Asmentioned above, most modern processes are operated under subatmosphericpressure, involving precipitation of the sodium sulfate as a saltcakewhich has to be removed from the reactor by filtering. Today it is hardto find any use for the saltcake and it is normally regarded as anunwanted by-product.

In order to avoid formation of a sulfate by-product, it has beendisclosed to provide all acid needed for the chlorine dioxide generationfrom chloric acid which can be prepared electrochemically from sodiumchlorate. Such methods are described in, for example, U.S. Pat. Nos.4,915,927, 5,084,148 and 5,174,868. However, it has been found difficultto achieve satisfactory current efficiency in production of strongchloric acid which is desirable in order to provide efficient chlorinedioxide generation.

U.S. Pat. No. 4,806,215 discloses a process in which chlorine dioxide isgenerated from sodium chlorate and hydrochloric acid, in which processthe generator liquor is acidified electrochemically and recycled back tothe reactor. However, this process necessarily results in co-formationof chlorine which cannot be accepted in modern environmentally friendlybleaching processes.

U.S. Pat. No. 4,129,484 discloses a process of producing chlorinedioxide in which process sulfuric acid and sodium hydrogen sulfate iswithdrawn from the reactor and subjected to electrolysis. However, thecurrent efficiency obtained in the electrochemical cell is notsatisfactory.

U.S. Pat. Nos. 5,198,080 and 5,122,240 disclose a process of producingchlorine dioxide involving crystallization and withdrawal of solidsodium sesquisulfate and optionally sodium chlorate. The solid salt isdissolved again, electrolytically acidified and recycled to the chlorinedioxide reactor. Since the process involves handling of solid materialit is fairly complicated. Further, the sulfate solution obtained bydissolving the solid sesquisulfate is fairly dilute.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide achlorine dioxide process of high efficiency and high production rate,wherein little or no chlorine is formed as a by-product.

It is another object of the invention to provide a chlorine dioxideprocess, as above, wherein no other detrimental by-products are formed,and wherein valuable by-products, such as alkali metal hydroxide,hydrogen gas and oxygen gas, are formed.

The objects of the invention are achieved by a process for continuouslyproducing chlorine dioxide at a pressure of about 400-900 mm Hg in anon-crystallizing process. Hydrogen peroxide is a preferred reducingagent. The acid normality in the aqueous reaction medium is from about 4to about 14 N and the chlorate concentration is between about 0.05mole/l to about saturation. No substantial amount of chloride ions isadded.

The objects of the invention are also achieved by a process forproducing chlorine dioxide comprising the steps of (a) providing areactor with an aqueous acidic reaction medium containing alkali metalchlorate and sulfate, wherein the concentration of sulfate exceeds about3 moles/l but is less than saturation; (b) reducing chlorate ions insaid reaction medium to form chlorine dioxide; (c) withdrawing chlorinedioxide gas from the reaction medium, (d) withdrawing reaction mediumfrom the reactor and transferring it to an electrochemical cell; (e)electrolyzing the reaction medium to increase the acidity and decreasethe content of alkali metal ions; (f) recycling the acidified reactionmedium to the reactor; and (g) adding make up alkali metal chlorate tothe reaction medium before or after the electrochemical cell; whereinthe process is performed substantially without crystallization ofsulfate or chlorate.

BRIEF DESCRIPTION OF THE DRAWING

For a full understanding of the invention, reference should be made tothe following detailed description and the drawing, wherein

FIG. 1 is a schematic illustration of one embodiment of the inventionemploying an electrochemical cell having three compartments; and

FIG. 2 is a schematic illustration of a second embodiment of theinvention employing an electrochemical cell having two compartments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

It has surprisingly been found that it is possible to produce chlorinedioxide safely with a high production rate and in a high yield in anon-crystallizing system. In one process alternative of the invention,hydrogen peroxide is used as the reducing agent and no substantialamounts of chloride ions are added to the reaction medium. In this firstalternative, the process is run at a pressure which can vary from 400 mmto 900 mm Hg absolute and is most preferably at atmospheric or slightlybelow (i.e., a slight vacuum). Under such conditions, it is notnecessary to add any chloride ions for a steady production. Neither isthere any problem with dangerous decomposition in spite of the reactionconditions being rather strong both in respect of chlorate concentrationas well as acid normality and temperature. The process is very efficientwith a yield of more than 94%. Since hydrogen peroxide is used as thereducing agent in this first alternative, no substantial amount ofchlorine is formed, nor are any other detrimental by-products.

In this first process alternative, it has also been found that theprocess has a lower cooling requirement than prior art processes. Due tothe hydrogen peroxide reduced process being a heat consuming reaction,the overall amount of heat that has to be removed by cooling is reducedconsiderably compared to, for example, the Mathieson or Solvay process.Further, by operating at a slight subatmospheric pressure theevaporation rate of the water is increased. Thereby the heat from thesystem is removed and the need for external cooling is further reduced.In particular, the objects of the invention are achieved in this firstalternative by a process for continuously producing chlorine dioxide byreacting an alkali metal chlorate, sulfuric acid or another chlorinefree mineral acid such as phosphoric acid, and hydrogen peroxide as areducing agent to produce chlorine dioxide in an aqueous reactionmedium, characterized in that the chlorine dioxide is generated in atleast one reaction step comprising at least one reaction vessel, byfeeding alkali metal chlorate, acid, hydrogen peroxide and inert gas tothe reaction vessel, in the substantial absence of added chloride ions,maintaining the aqueous reaction medium in the reaction vessel at apressure of from about 400 mm Hg to about 900 mm Hg, at a temperature offrom about 35° C. to about 100° C., at an acidity within a range fromabout 4 to about 14 N and at a chlorate concentration of between about0.05 molar to saturation, withdrawing chlorine dioxide, oxygen and inertgas and the depleted aqueous reaction medium from the reaction zonewithout substantial crystallization of alkali metal sulfate.

It has been found that the concentration of chlorate in the reactionsystem is very important. By raising the chlorate concentration tohigher levels than normal, high production rates and high yields areobtained. The increase of the production rate is as much as two to threetimes compared to the atmospheric pressure systems known in the art. Theability to use hydrogen peroxide as a reducing agent in an atmosphericpressure system with higher than normal chlorate concentration andwithout chloride ions was unexpected since the prior art teaches eithera combination of hydrogen peroxide and chloride or the use of verycautious reaction conditions. The high efficiency of the process alsobrings about the advantage of a low amount of produced by-product alkalimetal sulfate.

This first process alternative of the invention can be used in all typesof known reaction equipment for atmospheric pressure reactions. Thusreactors normally used for Mathieson or Solvay or R-2 processes can beused. In the process according to this embodiment, alkali metalchlorate, sulfuric acid, hydrogen peroxide and inert gas arecontinuously fed to the reaction vessel. Inert gas is added to thereaction vessel to obtain sufficient mixing and to maintain a safepartial pressure of the chlorine dioxide. With the hydrogen peroxidechemistry, oxygen is formed in situ as the reaction proceeds. The oxygenbubbles formed create both mixing and dilution of the chlorine dioxide.Thereby the flow of inert gas can be reduced compared to prior artprocesses. This also enables part of the inert gas to be introduced tothe top of the reaction vessel in order to decrease the foaming. Thus itis possible to reduce the amount of inert gas introduced into the bottomof the vessel. Usually the reaction is carried out in more than onereaction vessel. The depleted reaction medium is brought to a second (orsubsequent) reaction vessel with an additional amount of hydrogenperoxide and inert gas to further deplete the solution with respect toalkali metal chlorate.

In a preferred embodiment of the first process alternative, the processis carried out in two reaction steps. The first step comprises at leastone reaction vessel with reaction conditions as stated above. At least apart of the depleted aqueous reaction medium from the first reactionstep and alkali metal chlorate and hydrogen peroxide are brought to asecond reaction step comprising a single reaction vessel. The reactionmedium therein is maintained at a temperature of from about 50° C. toabout 100° C. and the acidity within a range of from about 2 to about 5N. The reaction medium is subjected to subatmospheric pressuresufficient for evaporating water. A mixture of chlorine dioxide, oxygenand water vapor is withdrawn from an evaporation zone in the reactionvessel and neutral alkali metal sulfate is precipitated in acrystallization zone in the reaction vessel. By combining a reactionstep which produces a depleted reaction medium with the SVP technology,the acid content of the depleted reaction medium from the first reactionstep can be used as acid medium in the SVP reaction vessel. As theprocess in the SVP reactor is run in a low acid normality range theprecipitated alkali metal sulfate from the SVP reactor will be neutral,which is an advantage. Thus, with this combination of reaction stepsthere will be no depleted reaction medium to take care of but only aneutral sulfate salt. The combination as described is known in the stateof the art as "cascading" of reaction vessels. However, it has notheretofore been made with hydrogen peroxide as the reducing agent. Theadvantages with "cascading" and hydrogen peroxide being a totallychlorine free system, a neutral salt by-product and a process with acommercially acceptable production also in the SVP reaction step. It iswell known in the art that SVP processes, with other reducing agents, inthe low acid normality range (2-5 N) are too slow to be of commercialinterest, at least without the aid of catalysts.

In a further embodiment of the first process alternative of the presentinvention, the depleted reaction solution from the first reaction stepis fed to an electrolytic cell. Such a process is known in the state ofthe art, e.g. from U.S. Pat. No. 4,129,484, the disclosure of which isincorporated herein by reference. Conventional cells could be used. Asuitable electrolytic cell is equipped with an anode and a cathode andpreferably at least one cationic membrane. Both FIG. 1 and FIG. 2,described in more detail hereinafter with respect to a furtherembodiment of the invention, illustrate such suitable electrolyticcells.

The depleted reaction solution from the chlorine dioxide reaction vesselis fed to the anolyte compartment of the cell and withdrawn as an acidenriched reaction solution which can be recirculated to the chlorinedioxide reaction vessel. In the cathode compartment alkali metalhydroxide is formed. Thus with this process it is possible to work upthe depleted reaction solution into two useful streams, one being theenriched acid solution which can be brought back into the system and theother being the alkali metal hydroxide. By this method the alkali metaladded to the chlorine dioxide reaction vessel can be withdrawn as auseful chemical instead of as a by-product. Depending upon the choice ofanode an additional cationic membrane can be used between the reactionsolution compartment and the anode. With an additional membrane it ispossible to avoid the oxidizing environment at certain anodes and toprolong the life time for the anodes. When an additional cationicmembrane is used the anolyte and membrane used are suitably insensitiveto the oxidizing environment at certain anodes.

It is also possible to feed the precipitated alkali metal sulfate formedas a by-product from an SVP reaction vessel to an electrolytic cell inthe same manner as mentioned above. In this embodiment the precipitatedsulfate is dissolved and added as a solution to the electrolytic cell.

The production of chlorine dioxide according to the first processalternative is performed by adding the alkali metal chlorate, sulfuricacid and hydrogen peroxide to the first reaction step. An aqueousreaction medium is maintained in the first reaction vessel with achlorate concentration of from about 0.05 moles/l to saturation,preferably from about 0.09 to about 3.75 moles/l, most preferably fromabout 0.09 to about 1.1 moles/l. The acidity in the reaction mediumshould be within the range of from about 4 to 14 N, preferably fromabout 6-12 N, and most preferably from 7.5-10 N. Hydrogen peroxide isadded in an amount of from about 0.16 to about 0.6 ton/ton chlorate,suitably from about 0.16 to about 0.32, preferably 0.16 to 0.22 ton/tonchlorate. Inert gas is introduced into the reaction vessel to provideagitation of the reaction medium and more importantly to dilute theproduced chlorine dioxide to a safe concentration. The amount of inertgas added is conventional, i.e. sufficient to maintain the partialpressure of the chlorine dioxide below about 100 mm Hg. The inert gasmay be air, nitrogen, carbon dioxide or the process off-gas comprisingoxygen and trace amounts of chlorine dioxide. The advantage of using theprocess off-gas as the inert gas is mainly that a relatively pure gascontaining a high concentration of oxygen is generated. A furtheradvantage is that the vent from the process will be reduced. Thus, it isa preferred embodiment to use the process off-gas as the inert gas.

The first reaction vessel is suitably operated at a temperature of 35°to 100° C., preferably from 45° C. to 70° C. and most preferably at 50°to 55° C. and at a pressure of from about 400 mm Hg to about 900 mm Hg,preferably from about 600 mm Hg to 800 mm Hg and most preferably fromabout 720 mm Hg to about 800 mm Hg. Thus it is preferred to operate atabout atmospheric pressure. The reaction is a non-crystallizing reactionand a depleted reaction medium leaves the reactor without anysubstantial crystallization of the alkali metal sulfate.

The first process alternative is an essentially chlorine free process.No substantial amount of chloride ions are added. To the contrary, ithas been found that chloride ions have a detrimental influence on theprocess and lead to a low hydrogen peroxide efficiency. The chlorateused in the process is conventional, commercially available chlorate. Byway of manufacture such chlorate always contains a small amount ofchloride. That amount of chloride is not more than about 0.5, often notmore than about 0.05, preferably not more than about 0.02, mostpreferably not more than about 0.1 weight percent of the alkali metalchlorate. Beside this amount of chloride being an impurity in thechlorate, no further chloride is added. There is also commerciallyavailable chlorate with higher amounts of chloride. This type ofchlorate has been obtained by adding extra alkali metal chloride to thechlorate. Such a chlorate is not suitable for the present process.

When the first reaction step is run in more than one reaction vesselsuch as in a conventional Mathieson process, the depleted reactionmedium from the first reaction vessel is brought to a second (orsubsequent) reaction vessel. Inert gas and hydrogen peroxide areintroduced and more chlorate in the depleted medium is converted tochlorine dioxide. The amount of added hydrogen peroxide in this secondreactor is suitably up to 50% of the entire hydrogen peroxide requiredfor reaction, and preferably only about 15% of the total amountrequired. The pressure in the second reactor is about atmosphericpressure and the temperature is the same as in the first reactionvessel. It is suitable to add heat to the second reactor to maintain thetemperature at the stated value. This heat may be added by an externalheater or by adding additional sulfuric acid to the second reactor.

In the non-crystallizing process inert gas is introduced in the reactionvessel, usually through the bottom, and chlorine dioxide and oxygen areliberated in the reaction medium. The introduction of inert gas bubblesin the medium as well as the release of gas bubbles in the reactionproducts brings about a tendency for foaming of the reaction medium,especially at higher rates. As mentioned above, inert gas is introducedinto the reaction vessel to dilute the chlorine dioxide formed to safeconcentration. The total quantity of inert gas that must be added isthen fixed depending on the value of the safe partial pressure of thechlorine dioxide formed.

However the method and mode of injection of the dilution inert gas isnot fixed. Normally all inert gas is introduced at the bottom of thevessel. In a preferred embodiment of the first process alternative ofthe present invention, only a portion of the inert gas needed isinjected in the bottom of the reaction vessel and through the reactionmedium. The rest of the inert gas is supplied to the space above theliquid level in the vessel. With this mode of introduction the foamingof the reaction medium can be reduced to a great extent or totallyeliminated. The proportion of inert gas supplied to the space above theliquid level is suitably 80% of the total amount of inert gas,preferably 50% and most preferably 20%. A suitable mode of arrangementof this divided inert gas supply must insure that the chlorine dioxideand dilution inert gas are well mixed in the vapor space above thereaction liquid. This can be achieved with multiple injection points,spargers, or baffles.

Another measure that can be taken to minimize the tendency of foaming isto increase the holes in the sparger through which the inert gas isintroduced.

In the embodiment with cascading the first reaction step with an SVPreaction vessel, at least a part of the depleted reaction medium fromthe first reaction step is brought to the single reaction vessel toprovide a reaction medium with an acid strength of from about 2 to about5 N. The reaction conditions in the vessel are suitably as stated inU.S. Pat. No. 5,091,166, the disclosure of which is incorporated hereinby reference. Thus, alkali metal chlorate concentration in the reactionvessel can vary within wide limits, from a low concentration of about0.25 M up to saturation, preferably from about 1.5 M up to saturation,most preferably from about 2.5 M up to saturation. Hydrogen peroxide canbe added in amounts of from about 0.16 to about 0.6 ton/ton chlorate,preferably from about 0.16 to about 0.32 ton/ton chlorate, and mostpreferably from 0.16 to 0.22 ton/ton chlorate. The reactants are addedcontinuously to the reactor. The reaction is suitably operated at atemperature of 50°-100° C., preferably 50°-75° C. and at a pressurebelow atmospheric pressure, suitably at 60-400 mm Hg. The reactionmedium boils or water is evaporated in an amount sufficient to dilutethe chlorine dioxide formed to a safe concentration. The acidity in thereactor is adjusted if necessary with extra sulfuric acid. In thisreactor the alkali metal sulfate formed in the total process iscontinuously crystallized and separated in at suitable manner. Thusinstead of a depleted reaction medium as a by-product to take care offrom the reactor the process in this embodiment produces a pure, neutralalkali metal sulfate.

The process is not restricted to any of the alkali metals, but sodium isthe most preferred.

In a second process alternative, an electrochemical cell is used inconjunction with a reactor using a wide variety of reducing agents andreaction conditions. The alkali metal can be any alkali metal but ispreferably sodium.

It is to be understood that the reaction medium entering theelectrochemical cell has substantially the same composition as it has inthe chlorine dioxide reactor. The reactor for generation of chlorinedioxide can be of any known type, such as SVP®, Mathieson and others,the reactor however being operated without crystallization.

It has been found that if the aqueous reaction medium is transferreddirectly to the electrochemical cell, without any intermediatecrystallization, it is possible to maintain a high content of sulfateduring the electrolysis. It has also been found that the currentefficiency during the electrolysis increases with the sulfate content ofthe reaction medium. Preferably the sulfate content exceeds about 4moles/l. The upper limit is determined by the concentration atsaturation which is dependent on several parameters, particularly theacidity. For instance, if the reaction medium has an acidity at about 1N, alkali metal sulfate starts crystallizing at about 5 moles/l, and ifthe acidity is about 6.5 N, the sulfate crystallizes at about 6.5moles/l. Most preferably, the process is operated at a sulfateconcentration just below saturation.

It has also been found that the current efficiency during theelectrolysis is improved if the molar ratio H⁺ :SO₄ ²⁻ is low. However,the solubility of sulfate also decreases with the above ratio. Further,the production rate of chlorine dioxide is improved if the acidity ishigh. The preferred content of H⁺ in the reaction medium is from about1.5 to about 11 moles/l, most preferably from about 3 to about 9moles/l. In order to obtain high efficiency both for the chlorinedioxide generation and the electrochemical acidification, it has beenfound that the optimal molar ratio H⁺ :SO₄ ²⁻ suitably is from about 0.5to about 1.5, preferably from about 0.7 to about 1.3.

In this second process alternative, the chlorate ions in the reactor aremost preferably reduced by a reducing agent, but electrochemicalreduction is also possible. Suitably, a reducing agent is added to thereaction medium, which reducing agent can be selected from organicsubstances such as methanol, ethanol, isopropanol, other alcohols orformaldehyde, or from inorganic substances such as hydrogen peroxide orchloride ions. Also mixtures of different reducing agents can be used.Hydrogen peroxide and methanol are the most preferred reducing agentssince they offer the possibility of efficiently producing chlorinedioxide substantially without formation of chlorine. A particularadvantage of using hydrogen peroxide is that a high production rate canbe achieved at low acidities, for example from about 2 to about 5 N, andthat no by-products that may damage the electrochemical cell areproduced.

The chlorine dioxide producing reactions are favored by the addition ofsmall amounts of catalysts to the reactor. Preferred catalysts belong tothe groups VB--VIII, IB, IVA and VIIA of the Periodic Table of theElements. High activity can be achieved by compounds containing V, Nb,Cr, Mn, Fe, Ru, Os, Ni, Pd, Pt, Cu, Ag, Ge, Sn, Pb, Br, and I, eitheralone or in combinations.

Although not necessary, it is possible to add small amounts of chlorideions, preferably in the form of alkali metal chloride, so as to maintainthe concentration thereof in the reaction medium within the range fromabout 0.001 up about 0.8 mole/liter.

Chlorine dioxide generation can take place under atmospheric pressure,i.e., within the range of pressure set forth above with respect to thefirst process alternative. The reaction medium may alternatively bemaintained under subatmospheric pressure in the reactor, which enableshigher concentration of chlorine dioxide without risk for explosion andalso improves the yield. However, contrary to conventionalsubatmospheric processes for chlorine dioxide production, no sulfate iscrystallized. Suitably, the absolute procedure is maintained from about60 to about 600 mm Hg, preferably from about 75 to about 400 mm Hg, mostpreferably from about 75 to about 350 mm Hg. However, it is preferred tooperate the electrochemical cell at atmospheric pressure, since pressurefluctuations in the different chambers may damage the membranes.

The temperature in the reactor is suitably from about 15° to about 100°C., preferably from about 40° to about 85° C. The temperature in theelectrochemical cell is suitable maintained at substantially the sametemperature as in the reactor.

Any suitable electrochemical cell enabling acidification of the reactionmedium can be used. Normally, a cell comprising an anode compartment anda cathode compartment divided by at least one ion selective membrane isbest suitable. In addition to an anode and a cathode compartment, such acell may comprise one or several compartments in the middle. Anystandard type of electrodes can be used. Further, standard polymericion-exchange membranes can be used, but also high ion conductingmembranes such as ceramic membranes can be useful. Normally, it ispossible to achieve a current efficiency of more than about 70% or evenmore than about 80%.

In one preferred embodiment of the second process alternative, thereaction medium to be acidified is supplied to the middle compartment ofa three chamber cell comprising two cation-exchange membranes.Preferably, water or an aqueous solution containing sulfuric acid issupplied to the anode compartment and water or an aqueous solutioncontaining alkali metal hydroxide is supplied to the cathodecompartment. In such a cell, hydrogen ions are generated in the anodecompartment and passed through the membrane into the middle compartmentreplacing alkali metal ions passed into the cathode compartment. In theanode compartment oxygen gas is produced, while hydrogen gas andhydroxide ions are produced in the cathode compartment. The advantage ofthis embodiment is that substances that may be present in the reactionmedium, such as chlorate, chloride ions and methanol, are not so easilyoxidized on the anode, thus avoiding formation of perchlorate, chlorineand formic acid. Further, the lifetime of the anode is increased.

It is also possible to perform the electrolysis in electrochemical cellsknown per se, for example, from a cell disclosed in the above-mentionedU.S. Pat. No. 4,229,487. Thus, it is possible to use a three chambercell in which the middle compartment is defined by an anion exchangemembrane and a cation exchange membrane, entering the reaction mediuminto the middle compartment, passing chlorate ions and sulfate ionsthrough an anion-exchange membrane into the anode compartment, andwithdrawing acidified reaction medium therefrom. Further, a two chambercell divided by an cation-exchange membrane could be used, acidifyingthe reaction medium in the anode compartment and passing alkali metalions through the cation-exchange membrane into the cathode compartment.In these cases, it is also possible to produce alkali metal hydroxide,hydrogen gas and oxygen gas as valuable by-products. It is also possibleto use a two chamber cell divided by a anion-exchange membrane. The mainadvantage of using a two chamber cell is that the investment costs arelower.

The method according to the invention can be performed substantiallywithout removing any chlorate or sulfate from the system. Substantiallyall chlorate supplied is transformed to chloride dioxide, i.e. the mainproduct. The alkali metal supplied can be withdrawn from the system asalkali metal hydroxide, a valuable by product. Sulfate is neither addednor withdrawn, but is circulating as a dead load, improving theefficiency of the electrochemical acidification of the reaction medium..Thus, it has been found possible to provide a method of producingchlorine dioxide from alkali metal chlorate without formation of byproducts other than valuable substances such as alkali metal hydroxide,hydrogen gas and oxygen gas. Another advantage of the invention, is thatonly a small amount of water has to be added to the system, thusdecreasing the amount that has to be heated and withdrawn byevaporation. Normally, water is only added to the system as a solventfor the make-up alkali metal chlorate and the reducing agent.

Since no salt is crystallized, there is no need for any filter whichsaves a considerable amount of the investment costs. Also, since everyalkali metal ion brings a couple of water molecules when passing themembrane in the cell, water will leave the system which involves theadvantage that less water has to be evaporated in the chlorine dioxidereactor, thus saving energy.

The invention will now be described more in detail with reference to thedrawings. FIGS. 1 and 2 schematically show two different embodiments ofthe invention. The invention is, however, not restricted to what isdescribed below, and it is apparent to those skilled in the art thatmany other embodiments can be employed within the scope of the claims.

Referring to FIG. 1, a preferred system for producing chlorine dioxideis indicated generally by the number 1 and comprises an SVP®-reactor 2containing an aqueous reaction medium 3. In the reaction medium 3chlorate ions, sulfate ions, hydrogen ions and sodium ions are present.A reducing agent R, preferably methanol or hydrogen peroxide, issupplied to the reaction medium 3 while chlorine dioxide generated inthe reaction medium 3 is withdrawn as a gas together with evaporatedwater thereby diluting the chlorine dioxide to a safe concentration. Inthe reactor 2, the absolute pressure is preferably from about 75 toabout 400 mm Hg and the temperature is preferably from about 50° toabout 85° C. If methanol is used as the reducing agent R, the reactionmedium 3 preferably contains chlorate in a concentration just belowsaturation, normally from about 4 to about 5 moles/l. Further, itpreferably contains from about 6 to about 7 moles/l of sulfate, fromabout 10 to about 12 moles/l of sodium, and has an acidity from about 6to about 7 N.

If hydrogen peroxide is used as the reducing agent, the chlorate contentof the reaction medium 3 is preferably just below saturation, normallyfrom about 4 to about 5 moles/l of sodium and has an acidity from about4 to about 5 N. The reaction medium 3 is continuously circulatingthrough line 4 and a heater 5. Part of the circulating reaction mediumis withdrawn from line 4 to line 6 and transferred to the centralcompartment 7 of a three chamber cell 8 comprising two cation-exchangemembranes 9, 10. In the cell 8, the anode compartment 11 is suppliedwith sulfuric acid from a tank 12 and the cathode compartment 13 issupplied with sodium hydroxide from a tank 14. In the anode compartment11, hydrogen ions are generated and passed through the membrane 9 intothe central compartment 7. Sodium ions from the reaction medium in thecentral compartment 7 are passed through the membrane 10 into thecathode compartment 13.

The electrochemical reactions result in acidification of the reactionmedium in the central compartment 7, generation of oxygen gas in theanode compartment, and generation of sodium hydroxide and hydrogen gasin the cathode compartment 13. The acidified reaction medium iswithdrawn from the central compartment 7 of the cell 8 through line 15,mixed with the reaction medium from the heater 5 and with an aqueoussolution of make up sodium chlorate, and then recycled back to thereactor 2. A portion of it can be recycled back to the cell 3 throughline 16. In the tank 12, oxygen is withdrawn and water is added to theanolyte. The three chamber cell 8 is operated under atmospheric pressureand the connections between the cell 8 and the reactor 1 is thereforeprovide with means 17, 18 for altering the pressure of the reactionmedium. In the tank 14, hydrogen and sodium hydroxide are withdrawn andwater is added to the catholyte. Means 17, 18 can consist of pumpsand/or valves for maintaining a pressure differential. Alteratively, thereactor 2 can be placed at a higher level than the cell 8, thusmaintaining a pressure differential by gravity (static head).

If the acidity in the chlorine dioxide reactor 2 is too low, theproduction rate decreases. On the other hand, if the acidity in the cell8 is too high, the current efficiency decreases. In order to achieveboth effective chlorine dioxide production and high current efficiency,the system comprises two recirculation loops for the reaction medium,one including the reactor 2, line 4 and the heater 5, the otherincluding the cell 8 and line 16. The acidity in the loop including thecell 7 cannot be lower than in the other loop, but it could be almostthe same. The difference in acidity of the medium in the two loopsshould be as low as possible, suitably below about 0.5 N, preferablybelow about 0.3 N, most preferably below about 0.1 N.

Referring to FIG. 2, another preferred embodiment for producing chlorinedioxide is indicated generally by the number 20. The system is similarto the one shown in FIG. 1, except that the electrochemical cell 21 onlyconsist of two chambers 22, 23 divided by a cation-exchange membrane 24.The chlorine dioxide reactor 2 and the catholyte system 23, 25 areoperated as in FIG. 1. The reaction medium to be acidified istransferred through line 26 to the anode compartment 22 of the cell 21,in which compartment 22 hydrogen ions and oxygen gas are generated.Sodium ions are passed through the cation-exchange membrane 24 into thecathode compartment 23 in which hydroxide ions and hydrogen gas aregenerated. The portion of the acidified reaction medium withdrawn fromthe anode compartment 22 not recycled back to the cell 21 through line27 is transferred to the chlorine dioxide reactor 28 in the same manneras in FIG. 1. Accordingly, chlorine dioxide, sodium hydroxide, hydrogengas and oxygen gas are produced as in the system described in FIG. 1.

The following Examples are intended to describe some specific ways ofoperating the process of the invention but should not be interpreted aslimiting its scope.

Examples 1-3 relate to the first process alternative using atmosphericpressure in the reactor and hydrogen peroxide as the reducing agent.Examples 4-7 relate to the second process alternative usingsubatmospheric pressure in the reactor and either methanol or hydrogenperoxide as the reducing agent.

EXAMPLE 1

To a laboratory chlorine dioxide generator a water solution of 64g/liter chlorate was added with 382 g/liter sulfuric acid. A chlorateconcentration of 0.38 M and an acid strength of 7.8 N was thusmaintained in the generator. 30% hydrogen peroxide solution was alsoadded such that the hydrogen peroxide concentration in the reactor was3.6 g/liter. The reactor was operated at atmospheric conditions andmaintained at 60° C. The chlorine dioxide production rate as 1.4×10⁻²moles/(liter-minute). The predicted chlorine dioxide production rate was1.44×10⁻² moles/(liter-minute).

EXAMPLE 2

To a laboratory chlorine dioxide generator a water solution of 64g/liter chlorate was added with 502 g/liter sulfuric acid. A chlorateconcentration of 0.13 M and an acid strength of 10.2 N was thusmaintained in the generator. 30% hydrogen peroxide solution was alsoadded such that the hydrogen peroxide concentration in the reactor was1.4 g/liter. The reactor was operated at atmospheric conditions andmaintained at 60° C. The chlorine dioxide production rate was 2.16×10⁻²moles/(liter-minute). The predicted chlorine dioxide production rate was2.64×10⁻² moles/(liter-minute).

EXAMPLE 3

A commercial generator of 1400 gallons of liquid capacity in the primaryreactor and 1400 gallons of liquid capacity in a secondary reactor inseries produced 10 tons of chlorine dioxide per day utilizingtraditional Solvay chemistry running at an acid normality of 9.5 N and achlorate concentration of 0.23 M at the overflow of the first reactor.Changing the reducing agent to hydrogen peroxide allowed a productionincrease to 18 tons of chlorine dioxide per day when operated at 8 N and30 tons of chlorine dioxide per day when operated at 10 N with achlorate concentration of 0.83 M in both cases at the overflow of thefirst reactor.

EXAMPLE 4

A 3 liter chlorine dioxide reactor was connected to a three chamberMP-cell™ (Electrocell AB, Sweden), forming a system as described in FIG.1 with a total volume of 5 liters. The system was charged with 5 litersof an aqueous solution consisting of 5.4 moles/l H⁺, 5.4 moles/l SO₄ ²⁻,1.9 moles/l ClO₃ ⁻ and 7.3 moles/l Na⁺. The anolyte was maintainedconstant at 100 g/l sulfuric acid by addition of water, and thecatholyte was maintained constant at 140 g/l sodium hydroxide bywithdrawing sodium hydroxide and feeding water. The chlorine dioxidegenerator was operating at 60° C. and 150 mm Hg, and the cell wasoperating at the same temperature but at atmospheric pressure. Methanolwas used as the reducing agent and the system was fed with a 545 g/lsodium chlorate solution. The cell was operating at a current of 30 A,corresponding to a current density of 3 kA/m² and the system wasoperating at steady state conditions for 5 hours. The current efficiencyfor sodium hydroxide production was 67% and the gram atom efficiency forchlorine dioxide production was 100%.

EXAMPLE 5

A 3 liter chlorine dioxide reactor was connected to a three chamberMP-cell™ (Electrocell AB, Sweden), forming a system as described in FIG.1 with a total volume of 5 liters. The system was charged with 5 litersof an aqueous solution consisting of 3.2 moles/l H⁺, 3.35 moles/l SO₄²⁻, 3.3 moles/l ClO₃ ⁻ and 6.8 moles/l Na⁺. The anolyte was maintainedconstant at 100 g/l sulfuric acid by addition of water, and thecatholyte was maintained constant at 140 g/l sodium hydroxide bywithdrawing sodium hydroxide and feeding water. The chlorine dioxidegenerator was operating at 65° C. and 195 mm Hg, and the cell wasoperating at the same temperature but at atmospheric pressure. Hydrogenperoxide was used as the reducing agent and the system was fed with a530 g/l sodium chlorate solution. The cell was operated at a current of30 A, corresponding to a current density of 3 kA/m², and the system wasoperated at steady state conditions for 8 hours. The current efficiencyfor sodium hydroxide production was 71% and the gram atom efficiency forchlorine dioxide production was 100%.

EXAMPLE 6

A 3 liter chlorine dioxide reactor was connected to a two chamberMP-cell™ (Electrocell AB, Sweden), forming a system as described in FIG.2 with a total volume of 5 liters. The system was charged with 5 litersof an aqueous solution consisting of 6 moles H⁺, 6 moles/l SO₄ ²⁻, 2moles/l ClO₃ ⁻ and 8 moles/l Na⁺. The catholyte was maintained constantat 140 g/l sodium hydroxide by withdrawing sodium hydroxide and feedingwater. The chlorine dioxide generator was operated at 60° C. and 150 mmHg, and the cell was operated at the same temperature but at atmosphericpressure. Methanol was used as a reducing agent and the system was fedwith a 545 g/l sodium chlorate solution. The cell was operated at acurrent of 30 A, corresponding to a current density of 3 kA/m², and thesystem was operated at steady state conditions for 8 hours. The currentefficiency for sodium hydroxide production was 66% and the gram atomefficiency for chlorine dioxide production was 100%.

EXAMPLE 7

A 3 liter chlorine dioxide reactor was connected to a three chamberMP-cell™ (Electrocell AB, Sweden), forming a system as described in FIG.2 with a total volume of 5 liters. The system was charged with 5 litersof an aqueous solution consisting of 4 moles H⁺, 4 moles/l SO₄ ²⁻, 2.2moles/l ClO₃ ⁻ and 6.2 moles/l Na⁺. The catholyte was maintainedconstant at 140 g/l sodium hydroxide by withdrawing sodium hydroxide andfeed water. The chlorine dioxide generator was operated at 65° C. and195 mm Hg, and the cell was operated at the same temperature but atatmospheric pressure. Hydrogen peroxide was used as a reducing agent andthe system was fed with a 530 g/l sodium chlorate solution. The cell wasoperated at a current of 30 A, corresponding to a current density of 3kA/m² and the system was operated at steady state conditions for 8hours. The current efficiency for sodium hydroxide production was 70%and the gram atom efficiency for chlorine dioxide production was 100%.

What is claimed is:
 1. A continuous, single vessel process for producingchlorine dioxide comprising the steps of:(a) feeding to a singlereaction vessel inert gas, and alkali metal chlorate, sulfuric acid orphosphoric acid, and hydrogen peroxide as reducing agent to form anaqueous reaction medium, said reaction medium being formed in thesubstantial absence of added chloride ions; (b) reacting the alkalimetal chlorate, acid and hydrogen peroxide to form chlorine dioxide,wherein during said reaction said aqueous medium in said single reactionvessel is maintained at a pressure of from about 400 mm Hg to about 900mm Hg, at a temperature of from about 35° C. to about 100° C., at anacidity within a range from about 4 to about 14 N and at a chlorateconcentration of between about 0.05 moles/l to saturation; and (c)withdrawing chlorine dioxide, oxygen and inert gas and the depletedaqueous reaction medium from said reaction zone said process beingconducted under conditions of non-crystallization of alkali metalsulfate.
 2. The process according to claim 1, wherein chlorine dioxideis produced at a pressure of from about 600 to 800 mm Hg.
 3. The processaccording to claim 1, wherein chlorine dioxide is produced at aboutatmospheric pressure.
 4. The process according to claim 1, wherein thechlorate concentration is between about 0.09 and about 3.75 moles/l. 5.The process according to claim 1, wherein the acidity is within therange of from about 6 to 12 N.
 6. The process according to claim 1,wherein the acidity is within the range of from about 7.5 to 10 N. 7.The process according to claim 1, wherein the depleted aqueous reactionmedium from the single reaction vessel being brought to at least asecond reaction step for further reaction with addition of hydrogenperoxide.
 8. The process according to claim 1, wherein a portion of theinert gas is injected in the bottom of the single reaction vessel andthrough the reaction medium and the rest of the air is supplied to thespace above the surface of the reaction medium in the vessel.
 9. Theprocess according to claim 1, wherein the inert gas is the processoff-gas.
 10. The process according to claim 1, wherein the inert gas isair.
 11. The process according to claim 1, wherein the depleted aqueousreaction medium from the single reaction vessel is fed to anelectrolytic cell.
 12. A continuous process for producing chlorinedioxide comprising the steps of:(a) continuously feeding to a singlereaction vessel inert gas, an alkali metal chlorate, sulfuric acid orphosphoric acid, and hydrogen peroxide as a reducing agent, to form anaqueous reaction medium, said reaction medium being formed in thesubstantial absence of added chloride ions; (b) reacting the alkalimetal chlorate, acid and hydrogen peroxide to form chlorine dioxide,wherein during said reaction said aqueous reaction medium in said singlereaction vessel is maintained at a pressure of from less than 760 mm Hgto about 400 mm Hg, and at a chlorate concentration of between about0.05 moles/l to saturation; and (c) withdrawing chlorine dioxide, oxygenand inert gas and the depleted aqueous reaction medium from said singlereaction vessel said process being conducted under conditions ofnon-crystallization of alkali metal sulfate.
 13. A process forcontinuously producing chlorine dioxide by reacting in at least twostages an alkali metal chlorate, sulfuric acid or phosphoric acid andhydrogen peroxide as a reducing agent to produce chlorine dioxide in anaqueous reaction medium, the process including the steps of:(a) feedingalkali metal chlorate, acid, hydrogen peroxide and inert gas in thesubstantial absence of added chloride ions to a first reaction stepconsisting of one reaction vessel; (b) maintaining the aqueous reactionmedium in said reaction vessel at a pressure of from about 600 to about800 mm Hg, at a temperature of from about 35° C. to about 100° C., at anacidity within a range from about 4 to about 12 N and at a chlorateconcentration of between about 0.05 molar and saturation; (c)withdrawing chlorine dioxide, oxygen and inert gas and the depletedaqueous reaction medium from said first reaction step said process beingconducted under conditions of non-crystallization of alkaline metalsulfate; (d) feeding at least a part of said depleted aqueous reactionmedium, alkali metal chlorate and hydrogen peroxide to at least a secondreaction step; and (e) recovering chlorine dioxide from said secondreaction step.
 14. The process according to claim 3, wherein the secondreaction step comprises a single reaction vessel, and wherein saidprocess includes the step of maintaining the reaction medium in saidsingle reaction vessel at a temperature of from about 50° C. to about100° C. and at an acidity within a range of from about 2 to about 5 Nand subjecting the reaction medium to subatmospheric pressure sufficientfor evaporating water, and wherein said step (e) of removing chlorinedioxide includes withdrawing a mixture of chlorine dioxide, oxygen andwater vapor from an evaporation zone in said single reaction vessel andprecipitating neutral alkali metal sulfate in a crystallization zone insaid single reaction vessel.
 15. The process according to claim 13,wherein the chlorate concentration in the reaction medium of said firstreaction step is maintained from about 0.09 to about 1.1 moles/l. 16.The process according to claim 13, wherein the amount of chloride ionsadded in the first reaction step is not more than about 0.5 wt. % of thealkali metal chlorate.
 17. The process according to claim 16, whereinthe amount of chloride ions added in the first reaction step is not morethan about 0.05 wt. % of the alkali metal chlorate.
 18. The processaccording to claim 17, wherein the amount of chloride ions added in thefirst reaction step is not more than about 0.02 wt. % of the alkalimetal chlorate.
 19. The process according to claim 18, wherein theamount of chloride ions added in the first reaction step is not morethan about 0.01 wt. % of the alkali metal chlorate.
 20. The processaccording to claim 13, wherein the temperature of the reaction medium inthe first reaction step is maintained from about 45° to about 70° C. 21.The process according to claim 13, wherein up to about 50% of the totalhydrogen peroxide required for the reaction is added to the secondreaction vessel.
 22. The process according to claim 13, wherein up toabout 15% of the total hydrogen peroxide required for the reaction isadded to the second reaction vessel.
 23. The process according to claim13, wherein the pressure in the second reaction step is aboutatmospheric.
 24. The process according to claim 13, wherein the chlorateconcentration is between about 0.09 and about 3.75 moles per liter. 25.The process according to claim 13, wherein the acidity for the firstreaction step is within a range of from about 6 to about 12 N.
 26. Theprocess according to claim 25, wherein the acidity for the reaction stepis within the range of from about 7.5 to about 10 N.
 27. The processaccording to claim 13, wherein a portion of the inert gas is injected inthe bottom of the reaction vessel of the first reaction step and throughthe reaction medium and the rest of the inert gas is supplied to thespace above the surface of the reaction medium in the vessel.
 28. Theprocess according to claim 13, wherein the inert gas is the processoff-gas.
 29. The process according to claim 13, wherein the inert gas isair.
 30. The process according to claim 13, wherein a portion of thedepleted aqueous reaction medium from the first reaction step is fed toan electrolytic cell.
 31. The process according to claim 1, wherein thechlorate concentration is between about 0.09 to about 1.1 moles/l. 32.The process according to claim 1, wherein the amount of chloride ionsadded is not more than about 0.5 wt. % of the alkali metal chlorate. 33.The process according to claim 1, wherein the amount of chloride ionsadded is not more than about 0.05 wt. % of the alkali metal chlorate.34. The process according to claim 1, wherein the amount of chlorideions added is not more than about 0.02 wt. % of the alkali metalchlorate.
 35. The process according to claim 1, wherein the amount ofchloride ions added is not more than about 0.01 wt. % of the alkalimetal chlorate.
 36. The process according to claim 1, wherein thetemperature of the reaction medium in the first reaction step ismaintained from about 45° to about 70° C.